We have previously shown that exposure of the eyes to bright light, but not ordinary room light can reverse the winter depressive symptoms in patients with Seasonal Affective Disorder (SAD). By now there are at least 40 controlled light treatment studies for SAD. Despite this, however, critics continue to argue that control treatments might have been implausible, which might account for the significant differences in efficacy between the active and control treatments. Since most light treatment studies performed to date have used full-spectrum light for the active condition, a novel way to re-examine the placebo issue would be to administer light of different colors. Basic science studies have shown that rod and cone photoreceptors are most sensitive to light in the green wavelength region. We therefore hypothesized that green fluorescent light would be an effective antidepressant treatment in SAD. Based on studies in animals, we hypothesized that red light would be inactive and would therefore serve as a suitable control. Previous studies by ourselves and others have shown that red light generates strong expectations of a positive outcome, which would fulfil the other parameter necessary for a good control treatment - plausibility. Additional benefits of a study in which red and green light were compared would be: 1) to help identify those photoreceptors important in the antidepressant response to light; 2) to clarify further whether ultraviolet light is necessary for its antidepressant effects. If this is not so, then this potentially toxic wavelength can be removed from the treatment without significant loss of efficacy. This past year we treated 14 SAD patients in a crossover study in which green and red fluorescent light, administered for two hours each morning, were compared. Green light proved to be superior to red despite equivalent a priori expectations of both types of light. Since the green light source contained no UV component, this finding provides additional evidence that UV light is not critical for the antidepressant effects of phototherapy.